Bi-directional multi-port inverter with high frequency link transformer

ABSTRACT

This invention is a multi-port power converter where all ports are coupled through different windings of a high frequency transformer. Two or more, and typically all, ports have synchronized switching elements to allow the use of a high frequency transformer. This concept and type of converter is known. This invention mitigates a number of limitations in the present art and adds new capabilities that will allow applications to be served that would otherwise not have been practical. A novel circuit topology for a four-quadrant AC port is disclosed. A novel circuit topology for a unidirectional DC port with voltage boost capabilities is disclosed. A novel circuit topology for a unidirectional DC port with voltage buck capabilities is disclosed. A novel circuit for a high efficiency, high frequency, bi-directional, AC semiconductor switch is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

More than one reissue application has been filed for U.S. Pat. No.7,102,251. This application is a divisional reissue application of priorU.S. patent Reissue application Ser. No. 12/205,743, titled“Bi-Directional Multi-Port Inverter With High Frequency LinkTransformer” and filed on Sep. 5, 2008, which is a reissue applicationof U.S. Pat. No. 7,102,251, which are incorporated herein by referencein their entirety.

BACKGROUND OF INVENTION

The field of this invention is power electronics and electrical powerconversion. Electronic power inverters are devices for converting directcurrent (DC) power, usually from a storage battery, into alternatingcurrent (AC) power for household appliances. Some inverters also convertpower from an AC source to charge the storage battery used by theinverter. Devices capable of power transfer in either direction,DC-to-AC or AC-to-DC are commonly referred to as inverter/chargers orbi-directional inverters. Inverters are also used in renewable anddistributed energy systems to convert DC power from photovoltaic panels,fuel cells or wind turbines into power that can be delivered into theutility grid. There is a growing demand for an inverter product withthis capability that can also charge storage batteries and support ACloads when the utility grid is not available. Residential systems withboth renewable energy sources and energy storage components typicallyuse a battery-centric topology. This is because the battery provides astable voltage and high peak power capabilities. In these systems, therenewable energy source interfaces to the battery through a DC-to-DCconverter or charge controller to provide the required matching andregulation functions. The battery is in turn connected to a DC-to-ACinverter, to support the system loads, and to a battery charger.Additional energy sources as well as DC loads would also logically tiein at the storage battery connection point. With the present state oftechnology, this arrangement typically provides the most cost effectiveand highest performance system solution. There are a number of inherentlimitations with this approach. (i) The storage battery voltages arerelatively low compared to the AC voltages that the inverter produces. Acommon power conversion method is to convert the low DC battery voltageinto a low AC voltage and then use a transformer to convert to a higherAC voltage. This approach requires a heavy, expensive, and typicallyinefficient, low frequency transformer. (ii) The conversion efficiencyfrom the renewable energy source to the battery to the utility grid islow because of the additive losses from each successive power conversionstage. (iii) Higher voltage, higher efficiency, lower cost photovoltaicseries “string” arrays are not practical because of thephotovoltaic/battery voltage disparity. (iv) Individual power convertersin battery-centric systems are usually autonomous. It is advantageousfor all power converters to act in concert in order to achieve optimumbattery life and to better support the system loads.

SUMMARY OF INVENTION

The invention is a multi-port power electronics topology, with a highfrequency transformer as the common power “conduit” and interface pointfor all ports. This invention would allow for energy systems that arehigh-frequency-transformer-core-centric as opposed to battery-centric.This invention mitigates essentially all of the limitations ofbattery-centric energy systems. The underlying power converter conceptused for this invention was originally invented by William McMurry anddisclosed in U.S. Pat. No. 3,517,300 in 1970. Since then, others haveexpanded the potential capabilities of these power converters but withless-than-novel or with technically obvious variations on the originalMcMurry invention. The invention disclosed herein involves a number ofnovel power circuit topologies that allow much greater port flexibilityand provide enhanced performance. The invention allows a port to performas a boost or buck converter when sourcing power into the high frequencytransformer, a capability that has not been previously established.These added capabilities allow applications to be served that wouldotherwise not have been practical. Also, the invention allows eachnon-battery port to “see” only the reflected battery characteristics atthe transformer interface so that the operation of all non-battery portsare independent and non-interactive. The preferred embodiment of theinvention is intended for residential electrical energy systems. Thereare three ports; a bi-directional battery port that allows a storagebattery to source energy to the transformer or sink energy from thetransformer to charge the battery, a bi-directional AC port that allowsthe transformer to source energy to loads and also to sink or sourceenergy from a utility grid at unity power factor, and a renewable energyport that sources energy into the transformer and is capable ofcontrolling the operating point of the renewable energy source and theamount of power delivered into the transformer. Products developed usingthis invention will be (i) lighter because transformers operating atultrasonic frequencies are much smaller than line frequency transformers(ii) lower cost because of the smaller transformer and thesystem-integrated power conversion approach and (iii) more efficientbecause of fewer power conversion stages and the lower core and copperlosses associated with high frequency transformers. These advantages arehad without sacrificing the isolation properties of a transformer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the preferred embodiment of the invention, a powerconverter for residential energy systems having a photovoltaic (PV)array, a storage battery and a multipurpose utility/load/generatorinterface.

FIG. 2 illustrates an alternate power converter circuit topology forresidential energy systems having a renewable energy source, a storagebattery and a multipurpose utility/load/generator interface.

FIG. 3 illustrates the sequence of high frequency switch closures in atwo-port power converter using the invention. The condition shown is abattery at the two-quadrant DC port delivering power to a load at thefour-quadrant AC port.

FIG. 4 illustrates the sequence of high frequency switch closures in atwo-port power converter using the invention. The condition shown is anAC voltage source at the four-quadrant AC port delivering power to(charging) the battery at the two-quadrant DC port.

FIG. 5 illustrates an alternate power converter topology for residentialenergy systems having a renewable energy source, a storage battery and asplit-phase, multipurpose, utility/load/generator interface.

FIG. 6 illustrates a typical, known bi-directional semiconductor switchcapable of bipolar voltage blocking, bi-directional current control andbi-directional current conduction.

FIG. 7 illustrates an alternate, bi-directional semiconductor switchcapable of bipolar voltage blocking, bi-directional current control andbi-directional current conduction using IGBT instead of MOSFET devices.

FIG. 8 illustrates a novel, composite, bi-directional semiconductorswitch capable of bipolar voltage blocking, bi-directional currentcontrol and bi-directional current conduction.

DETAILED DESCRIPTION

FIG. 1 illustrates the preferred embodiment of the invention, athree-port power converter topology with one bi-directional batteryport, at terminals 12 and 13, one four-quadrant AC port, at terminals 61and 62, and one unidirectional renewable energy port at terminals 84 and85. Two types of semiconductor switch elements are shown. Switches 21–24and 91–93 have unipolar voltage blocking, unidirectional current controland bi-directional current conduction capabilities and are referred toas unidirectional semiconductor switches on all diagrams. Switches 41–43have bipolar voltage blocking, bi-directional current control andbi-directional current conduction capabilities and are referred to asbi-directional switches in all diagrams. The battery port, at terminals12 and 13, contains a typical, full-bridge arrangement of power switches21–24 and is connected to winding 31 of high frequency transformer 30.Switch pairs 21, 23 and 22, 24 are alternately closed and opened at ahigh rate, typically greater than 20 kHz, providing the transformer withsquare wave excitation from a relatively low impedance source. Theswitching is free running and the duty cycle remains fixed at 50%. TheAC port, at terminals 61 and 62, contains a typical center-tapped,half-bridge switch topology using bi-directional semiconductor switches41 and 42. Switches 41 and 42 are always operated synchronously withswitch pairs 21, 23 and 22, 24 to basically unfold the high frequency ACsquare wave on windings 33 and 34. The flux in transformer 30 is alwaysreversed at the switching frequency of bridge 20. Unlike switch pairs21, 23 and 22, 24, switches 41 and 42 will operate at duty cycles fromzero to 50%, as commanded by a control circuit, to provide the desiredcurrent or voltage regulation for the AC port. The inclusion of switch43 allows the AC port to act as a boost circuit in, conjunction withinductor 51, when delivering energy from utility grid 66 tohigh-frequency transformer 30. Switch 43 also allows an efficient pathfor freewheeling inductor current when power is being delivered fromtransformer 30 to AC loads 64 or utility grid 66. Without switch 43, alimited boost function can be had by simultaneously closing switches 41and 42, causing transformer windings 33 and 34 to be short-circuited,and opening all bridge 20 switches. This works well for two-portconverters but limits the transformer availability for converters withthree or more ports because the transformer is unable to sink or sourcepower at any port when windings 33 and 34 are shorted. Also, anytimebridge 20 is not in conduction, the operation of one port becomesdependent on the operation of other ports and the value of this powerconversion approach is severely compromised. The inclusion and functionof switch 43 in the AC port is novel and part of this invention. Itshould be noted that if power is flowing into the AC port, inductor 51acts as a boost inductor, if power is flowing out of the AC port,inductor 51 acts as a filter component in conjunction with capacitor 52.The renewable energy port, at terminals 84 and 85, provides the abilityfor the converter to track the maximum power point of photovoltaic (PV)array 83 under various ambient conditions. The basic function of theport is that of a buck regulator. Energy from PV array 83 is stored incapacitor 55. Unidirectional switch 93 is turned on and off at a ratetypically greater than 20 kHz and with a duty cycle established by acontrol circuit to regulate the PV voltage and/or power. When switch 93is closed, diode 71 is back-biased and current flows through boostinductor 56 and returns through either transformer winding 37 andunidirectional switch 91 or through transformer winding 38 andunidirectional switch 92. When PV energy is available, switches 91 and92 always operate at 50% duty cycle and in tandem with switch pairs 22,24 and 21, 23 respectively. When switch 93 is opened, the freewheelinginductor current is conducted through diode 71 and either transformerwinding 37 and unidirectional switch 91 or transformer winding 38 andunidirectional switch 92, whichever path is active at the time. Thethree-switch buck port topology described here is novel and is part ofthis invention. FIG. 2 illustrates an alternate topology for therenewable energy port, at terminals 84 and 85. The basic function of theport is that of a boost regulator. In the preferred embodiment, therenewable energy source is either fuel cell 81 or DC generator 82.Energy from the renewable source is stored in capacitor 55.Unidirectional switch 93 is turned on and off at a rate typicallygreater than 20 kHz and with a duty cycle established by a controlcircuit to regulate the port voltage and/or power. When switch 93 isclosed, current flows from capacitor 55 to charge inductor 56. Whenswitch 93 is opened, the current flowing in inductor 56 is conductedthrough diode 71 and either transformer winding 37 and unidirectionalswitch 91 or transformer winding 38 and unidirectional switch 92,whichever path is active at the time. Switches 91 and 92 operate at 50%duty cycle and in tandem with switch pairs 22, 24 and 21, 23respectively, but may also be switched off when switch 93 is on. Thethree-switch boost port topology described is novel and is part of thisinvention. FIG. 3 illustrates one method of synchronizing the batteryport and AC port switching elements to convert power from a storagebattery to supply household AC loads. In this mode, AC voltage isregulated across the load. Regulation methodologies are known andtypically use voltage and current feedback, reference values and erroramplifiers to implement a fast inner current control loop and a slowerouter AC voltage regulation loop. FIG. 3 illustrates the sequence of acomplete high frequency switching cycle at point in time where a smallportion of the positive voltage half-sine across the load is beingcreated. In FIG. 3A, switch 41 is closed simultaneously with bridge pair21, 23 causing current to flow out of the battery and into the load inthe direction shown. In FIG. 3B, switch 41 is opened, interrupting thecurrent flow from the battery, and at the same time switch 43 is closed.Switch 43 acts as a freewheeling diode to provide a path for theinductor current. In FIG. 3C, bridge pair 21, 23 are opened and bridgepair 22, 24 is closed, at the same time switch 43 is opened and switch42 is closed. Current still flows through the load in the same intendeddirection even though the flux in the transformer has reversed. In FIG.3D, switch 42 is opened, interrupting the current flow from the batteryand at the same time switch 43 is closed, again providing a path for theinductor current. The sequence is then repeated 3A, 3B, 3C, 3D, 3A, etc.The ratio of switch 41 and 42 “on” times to the switching periodcontrols the amount of energy transferred and is effectively the PWMduty cycle controlled by the regulator. The selection of switch 41verses 42 controls the polarity of the voltage delivered to the load.The alternation of switch pairs 21, 23 and 22, 24 at high frequenciesenable the use of a high frequency transformer. FIG. 4 illustrates onemethod of synchronizing the battery port and AC port switching elementsto convert power from the AC utility grid to charge the storage battery.In this mode, AC current is sourced from the utility grid at unity powerfactor. The amplitude of the sine wave current out of the utility isproportional to the instantaneous battery charge current commanded bythe system controller's charge algorithm. Regulation methodologies areknown and typically use voltage and current feedback, reference valuesand error amplifiers to implement a current control loop with asinusoidal current reference that is synchronous with the AC linevoltage. FIG. 4 illustrates the sequence of a complete high frequencyswitching cycle at point in time where a small portion of a positivecurrent half-sine is being sourced from the utility grid. In FIG. 4A,switch 43 is closed and the inductor charges from the instantaneousutility line voltage. Bridge pair 21, 23 is closed but the states of thebridge pairs are irrelevant because switches 41 and 42 are both open. InFIG. 4B, switch 43 is opened and switch 41 is simultaneously closed. Theinductor current flows into the transformer. In FIG. 4C, bridge pair 21,23 are opened and bridge pair 22, 24 is closed, at the same time switch41 is opened and switch 43 is closed, charging the inductor. In FIG. 4D,switch 43 is opened and switch 42 is simultaneously closed and currentis again delivered to the transformer. The sequence is then repeated 4A,4B, 4C, 4D, 4A, etc. The ratio of switch 43 “on” time to switch 41 and42 “on” times controls the energy transferred. The transformer turnsratio is such that the battery cannot be charged from the utility gridunder normal conditions without the boost circuit. The selection ofswitch 41 verses 42 is selected based on the instantaneous AC linepolarity. In this battery charging mode, switch 43 provides a boostregulator function and switch pairs 21, 23 and 22, 24 operate assynchronous rectifiers. FIG. 5 illustrates two AC ports configured forinterface to a split-phase utility or to deliver power to split-phaseloads. FIG. 6 illustrates one method for configuring a switch elementwith the required characteristics for use as switches 41, 42 and 43 asreferencedin FIG. 1. Terminals 11 and 12 are the switch poles. The twoterminals are interchangeable with respect to any polarity reference.MOSFETs 7 and 8 are connected in a common source configuration so thatvoltage can be blocked in either direction and current flow can becontrolled in either direction. Gate driver 4 drives MOSFETS 7 and 8through resistors 5 and 6 respectively. MOSFETs 7 and 8 are switchedsimultaneously. The Vcc 2 to Vss 3 power supply and the logic drivesignal 1 are electrically isolated from the other switch elements in atypical power converter. A number of MOSFET devices may be paralleled sothat the conduction voltage drop of the MOSFET is always lower than theconduction voltage of the MOSFET parasitic diode. As such, current neverflows through the MOSFET parasitic diodes. The configuration shown inFIG. 6 is known. FIG. 7 illustrates a second method for configuring aswitch element with the required characteristics for use as switches 41,42 and 43 in FIG. 1. The method is essentially the same as shown in FIG.6 except that Insulated Gate Bipolar Transistors (IGBTs) are used inplace of FETs. This logical extension is obvious and thereforeconsidered known by default. FIG. 8 illustrates a hybrid switch thatincorporates the best features of both the MOSFET and IGBTbi-directional switches and is the preferred method for configuring aswitch element with the required characteristics for use as switches 41,42 and 43 in FIG. 1. Terminals 13 and 14 are the switch poles. The twoterminals are interchangeable with respect to any polarity reference.IGBTs 9 and 10 are connected in a common emitter configuration and eachare connected in parallel with MOSFETs 11 and 12 respectively. Voltagecan be blocked in either direction and current flow can be controlled ineither direction. Gate driver 4 drives all semiconductor devices throughgate resistors 5–8. The Vcc 2 to Vss 3 power supply and the logic drivesignal 1 are electrically isolated. In higher voltage applications, thehybrid switch illustrated in FIG. 8 operates with lower losses over awider range of currents than either the MOSFET only or the IGBT onlybi-directional switch. MOSFET devices exhibit a resistive “on”characteristic while IGBT devices exhibit a semiconductor junction “on”characteristic. In the AC port application discussed, the IGBT deviceshandle the high peak currents more cost effectively than the MOSFETdevices. High peak currents are shunted from the MOSFETS by the IGBTs.At lower currents, the current is shunted from the IGBTs and parasiticMOSFET diodes by a MOSFET “on” resistance that represents a lowervoltage drop than the semiconductor “on” voltage. Additionally, ifseparate drivers are used for the IGBTs and the MOSFETs, the MOSFETturnoff can be delayed with respect to the IGBT turnoff to takeadvantage of the faster MOSFET switching speeds. This bi-directionalhybrid switch is novel and is part of this invention.

1. A power converter apparatus comprising three or more ports, atransformer and a control circuit where one end of each port isconnected to a distinct winding on a common transformer core and wherethe remaining end of each port is connected to a load or power sourceand where each port comprises an arrangement of capacitive or inductiveenergy storage elements and semiconductor switches where individualsemiconductor switches are commanded on and off by said control circuitin a synchronous manner with semiconductor switches in other ports andwhere said power converter apparatus is further defined, as having oneport dedicated to a storage battery, designated for reference herein asthe battery port, having characteristics different from all other ports,specifically, semiconductor switches in the battery port operate in afree-running mode and provide frequency and phase references that arefollowed by synchronous switches in all remaining ports and theinterface at the battery port transformer winding is that of a lowimpedance AC voltage source or sink, whereas the interface at thetransformer windings of all other ports is that of a high impedance ACcurrent source or sink and where these two distinct port types, batteryand non-battery, enable energy transfer into or out of all non-batteryports simultaneously and in an autonomous manner in terms of energytransfer and where the net energy into or out of all non-battery portscharges or discharges the storage battery, respectively, via the batteryport.
 2. A power conversion method using a transformer comprising afirst winding, a second winding, and a third winding, a first switchingcircuit coupled to the first winding and to first terminals coupled to aphotovoltaic array, the first switching circuit including a buckregulator providing unidirectional energy flow from the photovoltaicarray to the transformer, a second switching circuit coupled to thesecond winding and to second terminals for connection to a battery, anda third switching circuit coupled to the third winding and to thirdterminals for connection to an AC power source connected to a utilitygrid or to a load, the method comprising: controlling the first, secondand third switching circuits by switching the first, second and thirdswitching circuits at a frequency much greater than a frequency of theutility grid such that at least some of the time, the first, second, andthird switching circuits are all active, with switching of the first,second and third switching circuits being synchronized with respect toeach other and such that when the second switching circuit is active toprovide energy from the battery, a voltage across the battery isprovided to the second winding; and selectively activating the buckregulator to cause unidirectional energy to flow from the photovoltaicarray to the transformer such that when the first switching circuit isactive no energy flows from the transformer back to the photovoltaicarray, wherein the buck regulator includes a capacitor and aparallel-connected diode connected across the photovoltaic array, aunidirectional semiconductor switch, and an inductor connected in serieswith the first winding, wherein the selectively activating causescurrent from the photovoltaic array to be converted to a correspondingvoltage at the capacitor of the buck regulator and then converted to acorresponding current via the inductor of the buck regulator andprovided to the first winding.
 3. The power conversion method of claim2, comprising switching the third switching circuit in accordance with avarying duty cycle so as to produce a line-frequency power waveform. 4.The power conversion method of claim 2, comprising controlling the firstswitching circuit so as to supply power to the transformer.
 5. The powerconversion method of claim 2, comprising controlling the secondswitching circuit so as to, at one time, supply power to the transformerand to, at another time, be supplied power from the transformer.
 6. Thepower conversion system of claim 2, comprising controlling the thirdswitching circuit so as to perform boost regulation.
 7. The powerconversion method of claim 2, comprising controlling the third switchingcircuit so as to, at one time, supply power to the transformer and to,at another time, be supplied power from the transformer.
 8. The powerconversion method of claim 2, comprising coupling the battery coupled tothe second terminals.
 9. The power conversion method of claim 2,comprising coupling the third terminals to the load or to the utilitygrid.
 10. The power conversion method of claim 2, wherein the frequencyof the switching of the first, second and third switching circuits is atan ultrasonic frequency or greater.